1. Field of the Invention
This invention relates to metrics of the quality of vision using wave front sensing and/or knowledge of the visual system. This invention particularly relates to new metrics for optical performance and a set of metrics correlated with visual performance.
2. Background of the Art
The advent of rapid, automated wave front sensing in the eye now provides the clinician with a much richer description of the optics of each patient's eye than has been available before. Numerous methods have been developed to measure the wave aberration, some of which are objective, such as the Shack-Hartmann wave front sensor, while others are subjective, such as the spatially resolved refractometer. In either case, these devices measure only optical characteristics of the eye. This is all that is required for some applications such as correcting the optics of the eye for imaging the retina. But in the case of correcting the optics of the eye for improving vision, neural processing as well as optical image formation is also important. As the technology for measuring the wave aberration matures, there is a need to discover better ways of using wave aberration measurements to improve vision. A key issue is how to transform the wave aberration into a succinct description of how it will affect the patient's vision.
The ability to predict the visual impact of a given wave aberration is important for several reasons. First, this information can be used to evaluate quality of vision and expected visual performance. Such information could be used in screening individuals for driver's licenses, disability claims, or evaluating quality of life issues such as the ability to recognize faces. Second, a metric derived from the wave aberration can guide the clinician in selecting the best strategy for improving vision in each patient. For example, are the higher order aberrations in the patient's wave aberration severe enough to warrant customized refractive surgery, or is she likely to benefit just as much from conventional refractive surgery? If the patient is complaining of haloes, flares, monocular diplopia, or other visual defects, can the problem be linked to the eye's optical performance, is the patient unusually sensitive to small defects in vision, or are other neural factors implicated? Third, metrics to predict the subjective impact of the wave aberration can be incorporated into algorithms to compute the best vision correction given a particular wave aberration. Methods of vision correction such as contact lenses, spectacles, and refractive surgery generally correct fewer aberrations than can be measured with wave front sensing technology. For example, spectacles can correct only prism, sphere, and cylinder whereas wave front sensors can reliably measure tens or even hundreds of aberrations in normal human eyes. The higher order aberrations can influence the values of defocus and astigmatism that provide the best subjective image quality. The development of metrics for subjective image quality that include the effects of higher order aberrations will allow the optimization of vision correction.
The common practice today is to rely on the patient's responses to refract the eye. These measurements are time-consuming with a typical subjective refraction taking several minutes per eye to perform. A wave front sensor measurement can be performed in a matter of seconds. A conventional subjective refraction involves adjusting three aberrations (sphere cylinder and axis) simultaneously to optimize visual performance. However, wave front technology allows many more than three aberrations to be corrected. A subjective procedure to identify the best values of more than three aberrations is not practical. For this reason, higher order corrections must depend on algorithms to optimize vision rather than on the subjective response of the patient. Moreover, conventional refraction is subject to the variability in the patient's response. If an objective metric could be developed that adequately mimics the behavior of the average visual system, one can average the results of multiple objective measurements in the time it takes to perform a single subjective refraction, reducing the variability in the correction and achieving a better visual outcome.
The role of individual aberrations in visual performance. Just as the conventional refraction can be decomposed into prism, sphere, cylinder, and axis, “irregular astigmatism” can be broken into individual aberrations, or Zernike modes, with a process called Zernike decomposition. Zernike decomposition can provide valuable insight into the relative importance of different aberrations for vision. It is useful in diagnosing the cause of a particular wave aberration as well as visual complaints. For example, a refractive surgery patient who presents post-operatively with an increase of vertical coma and complains of a vertical flare on car headlights at night very likely suffered some vertical decentration during laser ablation.
The evaluation of individual Zernike modes reveals large differences in their subjective impact. Applegate et al. (2002) created modified (or “aberrated”) log MAR acuity charts by convolving the image on a standard chart with the point spread functions (PSF) corresponding to individual Zernike modes. The visual impact of each Zernike mode in 2nd through 4th radial order was studied. A fixed level of RMS error (0.25 μm over a 6 mm pupil—a dioptric equivalent of 0.19 D) was used in each case. Subjects with 20/15 or better visual acuity and best corrected vision were asked to read each of the aberrated charts. The total number of letters read correctly up to the fifth miss were recorded for each chart. The number of letters lost was calculated by subtracting the number of letters read correctly for a perfect (unaberrated) chart. FIG. 1 shows the number of high contrast letters lost as a function of Zernike Mode. Note that more letters are lost for modes in the middle of a given Zernike order than those at either the beginning or the end of each order. For example, in the second radial order, defocus (labeled 4 in the figure) degrades performance more than either astigmatism mode (3 and 5). Similarly in the third radial order, coma (modes 7 and 8) decreases acuity more than trefoil (modes 6 and 9). Despite the fact that the total aberration as expressed by RMS error was constant, acuity varied by up to 10 letters (2 lines) depending on which Zernike mode contained the wave front aberration. FIG. 2 shows a simulation that captures the essence of Applegate's conclusion. The letter E at a size corresponding to (20/40) has been convolved with the PSF corresponding to each Zernike mode. The RMS wave front error of each Zernike mode was fixed at 0.25 microns. Wavelength was 555 nm and the pupil size was 6 mm. FIG. 3 shows the corresponding Zernike modes for comparison. Note that the letters at the center of the pyramid are more blurred than those along the flanks. Inspection of the original modes in FIG. 3 shows why this is true. The flanking modes share the common feature that the wave aberration is flat (uniform gray in the figure) over much of the pupil. The light that passes through these regions of the pupil will form sharp images on the retina. The aberrations that blur strongly, on the other hand, tend to have nonzero slope over a larger contiguous fraction of the pupil.
Chen and Williams obtained similar results to those of Applegate et al (2002), using a deformable mirror to produce aberrations instead of using the MAR acuity charts, modified by image processing convolution with the point spread functions. They used the deformable mirror to blur the subject's vision with a single Zernike mode, one at a time, while all other aberrations were corrected across a 6 mm pupil. The subject adjusted the coefficient associated with this Zernike mode to produce an amount of blur that equaled a standard amount of blur. They also found that aberrations in the center of the pyramid blurred more than those at the edge, and that this was true for 5th order aberrations as well as 2, 3, and 4th. See FIG. 3 to see which modes belong to each order.
The Problem with Zernike Decomposition. By analogy with the success in chemistry of reducing molecules to their atomic constituents, it is tempting to think that reducing the wave aberration to its fundamental components might provide the path to subjective image quality. However, experiments cast doubt on the value of this reductionist approach because Zernike modes can interact strongly with each other to determine final image quality. Their subjective effects do not add together in a simple way as illustrated in FIG. 4. Shown are the retinal images of the letter E for three hypothetical eyes, one suffering only from defocus, one suffering from spherical aberration, and one suffering from both defocus and spherical aberration in the same amounts as present in the first two eyes. (In the usual practice to describe the effect of multiple aberrations, it is the variance, which is the RMS squared, which is added, not the RMS itself. For example, in this case 0.252+0.142=0.2872). Strikingly, the image quality is obviously best in the eye that suffers from both aberrations rather than the eyes that suffer from only one of them. Consistent with this demonstration, Applegate et al. (2003) have measured the interactions between Zernike modes and found that pairs of aberrations can sometimes increase acuity more than would be expected from the individual components or they can sometimes lead to a larger reduction in acuity than expected (Applegate et al., 2003). Modes two radial orders apart and having the same sign and angular frequency (e.g., C20+C40) tend to combine to increase visual acuity compared to loading the same magnitude RMS error into either component individually. Modes within the same radial order (e.g., C4−4+C40) tend to combine to decrease acuity compared to loading the same magnitude RMS error into either component individually. The complexity of the interactions between Zernike modes in subjective blur means that Zernike decomposition is unlikely to be a productive avenue for deriving a metric of subjective image quality. However, our visual quality metric derived from wavefront and visual processing can predict subjective image quality (Marsack et al 2004).